I have stayed at both many times. The Renaissance has a private island with a shuttle (boat) taking you there. NO ONE ELSE (but Renaissance visitors) can get there (room key is your "boat pass"...so it is not crowded at busy times. The Renaissance has two buildings (Marina tower - adult only -- with an infinity edge pool, overlooking sunset and cruise port) and another buillding -- which actually has 2 pools and a little "beach" (sand -- but separated from the Caribbean by a breakwater). BOth buildings are a bit dated in terms of decor -but room size is great!
The Marriott is on Palm Beach - the most crowded, but popular beach on the island. it has been recently renovated....and is very nice....the access to shade at the beach is limited (as with many Palm beach hotels)..you need to either reserve them with $$ or get up early for signing up for one.
Postscript (3/2010) the Renaissance Aruba is one of the properties targeted to move down one level in terms of marriott point redeption -- as a category 5 (which it will move to) it's a great bargain -- try it!!!
Both are near plenty of "night life" now -- a shopping mall has gone up near (walking distance) of the Marriott..with movies and upscale shops and restaurants. The Renaissance is in the "thick of things" downtown. Each hotel has a casino.
Actually you would probably enjoy either one -- if you want modern rooms and a beach out your front windo -- then the Aruba Marriott is your choice...if you want a more European feel, larger rooms and plenty you can walk to downtown..then the Renaissance will do fine. Both have very good GM's ...